(1) Field of the Invention
The present invention relates to a stepper motor apparatus and a method for controlling a stepper motor, and particularly relates to a stepper motor apparatus for driving a pointer and a method for controlling the stepper motor.
(2) Description of the Related Art
Conventionally, a stepper motor apparatus has been found to be suitable for use in, for example, an automotive dashboard indicator. The indicator includes a dial having graphical indicia and a pointer which is driven around the dial. The stepper motor apparatus has been used as a device for driving the pointer.
This type of stepper motor apparatus provides a driving signal a duty cycle of which is modulated, and therefore alters the excitation state of an excitation coil inside the stepper motor periodically. As a result, a drive torque will be produced inside a magnetic rotor surrounded by the excitation coil, resulting in rotation of the stepper motor.
More specifically, during the stepper motor apparatus, the driving signal of the phase corresponding to the difference (θ−θ′) between an commanded position (θ) and an actual position (θ′) of the pointer is fed into the stepper motor, in order to rotate stepper motor (θ−θ′) degrees. Both the commanded position (θ) and the actual position (θ′) of the pointer have been stored in memory inside the stepper motor apparatus. In the case where the stepper motor apparatus initializes the actual position of the pointer on the basis of the driving signal which is fed into the stepper motor, it also initializes the commanded position of the pointer on the basis of the values each of which is measured and displayed by a variety of sensors. Thereby, the pointer can be driven around the dial having graphical indicia.
Meanwhile, during the indicator, due to the input of the value measured by means of a variety of sensors which vibration or noise of vehicles can adversely affect, the actual movement of the pointer may be different from its measured movement, i.e., (θ−θ′). If this phenomenon is repeated, the actual position θ stored in memory can be different from the position displayed on the dial, resulting in an inaccurate indication.
To solve the above-discussed problem, a stepper motor is provided with both a protrusion and a stopper. The protrusion is, for example, a projection portion which is formed on a gear of the pointer for transmitting rotation of the stepper motor to the pointer, and therefore is associated with the stepper motor. For this reason, the term protrusion described herein is equivalent to a driven member and these terms are interchangeably used herein. When the stopper is in abutment with the protrusion, the stepper motor's rotation is obstructed by the stopper. In the case where the stopper is in abutment with the protrusion, the pointer is positioned at a zero point (it is also designated as a reference point).
Further, during the stepper motor apparatus, whenever an ignition switch is switched on so as to supply power thereto, the driving signal is fed into the stepper motor such that the stepper motor is made to rotate in the backward direction where the protrusion is driven toward the stopper. As a result, the protrusion is forced to abut the stopper, and thereby the pointer is compulsively stopped at the zero point. Finally, the stepper motor apparatus can reset the actual position (θ′) stored in the memory to the zero point.
In the case where the actual position (θ′) stored in the memory is reset to the zero point, since the pointer on the dial also indicates the zero point, it is possible to reset the difference (θ−θ′) between the actual position stored in the memory and the position of the pointer displayed on the dial.
Moreover, the above discussed abutment detection may be carried out, for example, by examining the voltage of on both ends of detection coils where induced voltage generates in response to rotation of the magnetic rotor. For example, the voltage levels of both ends of the detection coils are higher than a threshold voltage level, which indicates that the magnetic rotor rotates while generating a high level of induced voltage, and the protrusion is not in abutment with the stopper.
On the other hand, when the voltage level is lower than a threshold voltage level, the induced voltage is hardly generated, and the magnetic rotor is found to be obstructed mechanically by the stopper being in abutment with the protrusion. Additionally, the use of an excitation coil, which has been controlled in a non-energized manner, as a driving coil during rotation of the stepper motor is disclosed in Japanese Publication No. 2003-125599.
The position where the protrusion is in abutment with the stopper depends on a thermal expansion or an aged deterioration of each component comprising the stepper motor apparatus under the ambient temperature. Therefore, when the output of the driving signal corresponding to the amount of difference (θ−θ′) is intended from the phase of the driving signal at the point of detecting that the protrusion is in abutment with the stopper, a problem occurs that the indication of the pointer may be varied depending on the aged deterioration or the thermal expansion of each components comprising the stepper motor apparatus due to a change in ambient temperature.
Thus, a need exists for an improved stepper motor apparatus for preventing the indication of the pointer from varying even if a change in the position where the protrusion is in abutment with the stopper occurs, and a method for controlling the stepper motor.